Re: [Pce] Whither Stateless PCE?

["Zhangxian (Xian)" <[e-mail]>]

Hi,

   I would like to first thank Olivier and Mustapha for bringing such good comments/discussions, based on inter-op test.  With your help, hope we can clear the ambiguity very soon.

With regard to the 2nd point raised below by Mustapha:
＂
In addition, I think it is worth considering sending the constraints in a PCRpt message in duplicate Metric/LSPA objects with the P-flag set. This is in addition to the same objects containing the operational values. This can be useful in the case where the initial path was computed by the router and it is active and the user is delegating it. The PCE at that point in time will not compute a path immediately but will save the original constraints in the PCRpt message for the next time it needs to update the path.＂

The capability of carrying Metric/LSPA in PCRpt message is already allowed (see https://tools.ietf.org/html/draft-ietf-pce-stateful-pce-14#section-6.1).  If you think there is lack of text explanation of the motivation behind, we should add some to make it clearer.

Regards,
Xian

From: Pce [mailto:[e-mail]] On Behalf Of Aissaoui, Mustapha (Nokia - CA)
Sent: 2016年4月19日 4:00
To: EXT Olivier Dugeon; [e-mail]; 'Dhruv Dhody'
Cc: [e-mail]
Subject: Re: [Pce] Whither Stateless PCE?

Hi Olivier,
It is one option for sure. In general, implementations of stateful PCE should be able of caching the constraints received in the PCReq message for some period of time to give a chance for a potential follow-up PCRpt message. Even if you set the D-flag in the PCReq message, there is no guarantee that a PCRpt will follow and as such a PCE implementation will have to flush that information from its cache at some point in time.

In addition, I think it is worth considering sending the constraints in a PCRpt message in duplicate Metric/LSPA objects with the P-flag set. This is in addition to the same objects containing the operational values. This can be useful in the case where the initial path was computed by the router and it is active and the user is delegating it. The PCE at that point in time will not compute a path immediately but will save the original constraints in the PCRpt message for the next time it needs to update the path.

Regards,
Mustapha.

From: EXT Olivier Dugeon [mailto:[e-mail]]
Sent: Monday, April 18, 2016 8:58 AM
To: Aissaoui, Mustapha (Nokia - CA); [e-mail]; 'Dhruv Dhody'
Cc: [e-mail]
Subject: Re: [Pce] Whither Stateless PCE?

Dear Mustapha,

You catch a good point regarding the original constraints that are not carry by the PCRpt message. Thus, if we used a standard PCReq message without the D-delegate flag set, there is a risk that the PCE considers this request as a stateless one and don't keep track of the original request, and consequently, original constraints.

So, is it preferable to set de D-delegate flag in the PCReq message to tell the PCE to keep in memory the original constraints for further usage, or, is the 'STATEFUL-PCE-CAPABILITY' TLV in Open message is sufficient for the PCE to know that it must keep track of any requests? I prefer the first option as it allows a per request configuration while the second enables the memorization globally for all requests.

Regards,

Olivier
Le 08/04/2016 19:26, Aissaoui, Mustapha (Nokia - CA) a écrit :
Hi Olivier,
Good summary indeed. I was worried about interop testing when I sent the original email to the list in December 2014.

I just wanted to comment on a couple of things:


1.     You are correct that the LSP object which has the D-delegate flag is allowed in the PCReq message as per draft-ietf-pce-stateful-pce. I however think it is more appropriate to do the delegation in the subsequent PCRpt message once the LSP path is programmed by PCC following the PCRep message from PCE. This is because it is at that time that the LSP is being synchronized with the PCE LSP database.



2.     The PCRpt message does not carry the original constraints of the LSP (Bandwidth, Metric, and LSPA objects). It can carry the operational values of the Bandwidth and Metric objects used by the last computed path in the router. So, even if you have a PCE which reacted to the PCRpt message and computed a new path, it will not get the appropriate constraints included. That is why the PCReq/PCRep sequence before delegating the LSP is needed.

Regards,
Mustapha.

From: EXT [e-mail]<mailto:[e-mail]> [mailto:[e-mail]]
Sent: Friday, April 08, 2016 12:29 PM
To: Aissaoui, Mustapha (Nokia - CA); [e-mail]<mailto:[e-mail]>; 'Dhruv Dhody'
Cc: [e-mail]<mailto:[e-mail]>
Subject: Re: [Pce] Whither Stateless PCE?

Hello all,

IMHO the discussion must be split into is 2 different subjects:

1/ PCInit message could be seen as an independent message compared to other PCReq/PCRep, PCRpt and PCUp. Indeed, the PCE uses the PCInit message after a request that comes from another interface (e.g. a RestConf API) instead of PCReq that comes from the router itself through PCEP. In fact, when you configure a tunnel on the router, only the path computation part is requested to the PCE. Complements of tunnel configuration still remain in the router configuration. In case of PCInit, all information must be provided to the router. This could be for example the traffic steering. So, IMHO, it is normal that the PCInit message evolves through extensions different from the other PCEP messages, and in particular PCReq, as it is not triggered by the same entity, i.e. an external component instead the PCC router itself.

2/ But, this will not make PCReq message obsolete. Indeed, RFC5440 will continue to be mandatory for stateful both passive and active mode even if it needs clarification in the draft. Let me explain. In passive stateful, a PCReq/PCRep sequence is drawn in Figure 7 of the pce stateful draft prior to the PCRpt message Now, the ambiguity comes from the active stateful mode and figure 8. Why is the PCReq/PCRep sequence not mentioned? Of course the tunnel is delegated in this mode, but, the delegation object has been added as an extension to the PCReq message in the same draft. So, IMHO, at the creation of the tunnel, the draft must precise that a PCReq/PCRep exchange with delegation=1 must be used prior to the PCRpt to be coherent with RFC 5440 and passive stateful mode.

The problem occured during our evaluation of commercial products on which we made interoperability tests. Indeed we observed different behaviours that are due to the draft ambiguity and conduct to some interoperability issues. The different cases are as follow:
 - a/ - PCReq/PCRep exchange to obtain a valid ERO before the PCRpt message
 - b/ - PCReq message to obtain a valid ERO but with no reaction from the PCE which is not conform to RFC5440
 - c/ - PCRpt with empty ERO (looks strange. What is the meaning of an Empty ERO ? a loose path ? no path ? )/PCupd to get a valid path which overlaps with standard RFC5440 PCReq/PCRep.
 - d/ - PCRpt with empty ERO and no PCUpd leaving the tunnel down.

Thus, PCC/PCE that used PCRpt/PCupd messages for active stateful mode are incompatible with PCC/PCE that used standard PCReq/PCrep exchange. We could not mix both behaviours (PCC that use PCReq message with PCE that react to PCRpt with empty ERO and reciprocally). The problem occurs only at the creation of the tunnel. Once created and up the tunnel is reported and updated by means of PCRpt / PCupd messages correctly in all cases.

To summarize: PCInit message could leave independently from other messages. PCReq is the basis of PCE and is mandatory in all use cases included the active stateful mode, but this need to be clarify in the pce stateful draft.

Regards

Olivier
Le 07/04/2016 23:22, Aissaoui, Mustapha (Nokia - CA) a écrit :
Hi Adrian,
I raised in December 2014 the technical issue in draft-ietf-pce-stateful-pce that a PCC must be able to convey the original parameters (constraints) of the LSP path (Bandwidth, Metric, and LSPA objects) using a PCReq message to a PCE and subsequently delegate the LSP to PCE using the PCRpt message. Otherwise, when the LSP is delegated to PCE only the operational values of these parameters can be included in the PCRpt message. The latter means that the PCE will update the path without knowing exactly the original parameters.

For me, PCReq/PCRep are an integral part of operating an LSP in stateful mode.

Here is the link to the archived thread:
https://mailarchive.ietf.org/arch/search/?email_list=pce&so=-date&q=%22+Path+Computation+Request+in+Active+Stateful+PCE%22

Regards,
Mustapha.

From: Pce [mailto:[e-mail]] On Behalf Of EXT Adrian Farrel
Sent: Thursday, April 07, 2016 12:48 AM
To: 'Dhruv Dhody'
Cc: [e-mail]<mailto:[e-mail]>
Subject: Re: [Pce] Whither Stateless PCE?

I think you are probably right, Dhruv.

But referencing the ways in which customers deploy may be a little limiting.
To say PCE is widely deployed (even after all these years) may be an exaggeration.
Although we do have some clues about what is currently being pushed for deployment.

I think you have mainly grasped my point, however. We need to understand which extensions are definitely only needed in one mode or another, and which should be done in all modes (either because they are needed or because we don't know).

OTOH, I suppose TLVs are just TLVs. Once you specified the TLV it is not rocket science to include it in a message. In fact, it is probably one line of text to include it and only a short paragraph to describe additional processing in other modes once you have described how it is used in one mode.

Where does that leave us?

Adrian

From: [e-mail]<mailto:[e-mail]> [mailto:[e-mail]] On Behalf Of Dhruv Dhody
Sent: 06 April 2016 23:07
To: Farrel Adrian
Cc: [e-mail]<mailto:[e-mail]>
Subject: Re: [Pce] Whither Stateless PCE?

Hi Adrian,

Even in the brave new world of Stateful PCE, PCReq and PCRep messages do play a role in the passive stateful PCE mode. PCReq/PCRep also play a crucial role in the inter-domain and inter-layer context in the new proposal like stateful H-PCE.

At the same time mandating that every extension (say SFC) must also be specified in a stateless manner when no customer deploy in such a way, might be overkill.

Perhaps we need to look at it case by case!

Dhruv

On Wed, Apr 6, 2016 at 4:00 PM, Adrian Farrel <[e-mail]<mailto:[e-mail]>> wrote:
Once upon a time, in a working group far, far away, PCE was basically stateless.
PCE acted in response to questions asked by PCCs.

These days, everyone is excited by stateful PCEs and there is a lot of
initiation (of LSPs or of control of LSPs).

In the jabber room during today's meeting Ravi noted that not a lot of the new
drafts (maybe none of them) talk about PCReq messages. This raises the question
in our minds as to whether stateless PCE is obsolete.

If (and only if) this mode of PCE usage has gone out of fashion, we *might*
consider cleaning up the protocol and architecture so that we don't need to make
protocol extensions to PCReq and PCRep messages when we make extensions to
PCInit messages.

Thoughts?

Adrian

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